The present invention relates to a process for the production of sandwich elements (so-called panels) in which the ascending reactive mixture is led, when it rises, through guide elements arranged between the upper and lower outer layers, and in this way void-free panels having a regular cell structure can be produced. The invention also relates to an apparatus for their production.
The production of sandwich elements (panels) from reactive plastics material is generally carried out in a continuous process. In this connection, the panels are produced in an endless manner on devices such as that which is commercially available from Hennecke under the name Contimat, in thicknesses of 10 to 200 mm. Such a device normally consists of a circulating upper belt for guiding the upper outer layer, a circulating lower belt for guiding the lower outer layer, a feed device for the upper outer layer, a feed device for the lower outer layer, a forming section within which the reactive mixture, e.g. a polyurethane reaction mixture or polyisocyanurate reaction mixture, foams between the upper outer layer and the lower outer layer and fully reacts, a device for cutting the produced panel into the desired lengths, as well as a metering station with a mixing head for applying the polyurethane reaction mixture to the lower outer layer. An example of such a prior art device is shown in FIG. 1.
In this connection, the liquid reactive mixture is applied to the lower belt i.e. to the lower outer layer. The application is effected by means of a mixing head running transverse to the transporting direction. Since the mixture applied in this way is not completely flat, the reactive mixture rising due to the blowing gas that is subsequently produced forms an irregular and uneven (rough) surface forms. When this reactive mixture with the uneven (rough) surface reaches the upper belt, i.e. the upper outer layer, air is trapped between the uneven surface or humps and the upper belt. Panels with voids are thus formed. Such panels are unsuitable for use in technical applications since they have clearly recognizable voids, and because such panels are significantly distorted due to the voids and they become crooked. Panels with voids on the upper side are formed especially if the production is, as the practitioners say, too fast (i.e., the transporting velocity of the outer layers for the instantaneously metered amount of reactive mixture is too high). Such an operating mode corresponds to the rise curve a of the reactive mixture as shown in FIG. 2.
In practice, it has now been found that surface voids can be avoided by lowering the production rate, in other words by lowering the transport velocity of the outer layers for the same metered amount of reactive mixture. In this case, the foam front between the outer layers exhibits a so-called bow wave. Such an operating mode corresponds to the rise curves b and c in FIG. 2. This so-called bow wave is characterized by a depression or hollow formed in the transporting direction of the outer layers caused by the foam front rising between the outer layers over the height of the reactive mixture. In such an operating mode, the irregular surface of the rising reactive mixture no longer occurs.
The rise curve b illustrated by way of example in FIG. 2 corresponds to an operating mode with a production rate driven relatively too slowly. Although the occurrence of voids is avoided in this case, significant “overrolls” are formed in the panel.
The essential physical data that determine the quality of a panel are the compressive strength and the insulating effect.
During the development of propellant gas in the reactive plastics material, cells are formed that are elongated, i.e. substantially oval, corresponding to the direction of ascent. Panels with a homogeneous cell structure in which the cells are aligned vertically (i.e., substantially perpendicular to the panel outer layers) are now desired. This is because the loading capacity of cells in the longitudinal direction (i.e., parallel to their long sides) is significantly higher than transverse thereto. Accordingly, panels in which the substantially oval cells are formed perpendicular to the outer layers have the best compressive strength values. In this connection, the number of cells is normally in the range from about 10 to about 100 cells per cm, i.e., the height of a vertically aligned cell is generally of the order of magnitude of about. 0.1 to 1 mm.
The same applies to the insulating effect. The smaller the heat conductor cross-section and the longer the heat conductor paths of the cell membranes in the plastics matrix, the better the insulation. This also means that a cell structure with cells aligned substantially vertically to the outer layers represents the optimum structure. This in turn means that overrolls, such as occur in a procedure corresponding to the rise curve b illustrated in FIG. 2, leads to a cell structure with completely randomly arranged cells, i.e. to cells that for a large part do not lie perpendicular to the outer layers. This in turn means that such panels exhibit too low an insulating effect and also have too low a compressive strength, particularly on the upper side of the panel.
A compromise which has been adopted in practice is to drive a production rate (i.e., adjust a transporting velocity of the outer layers) that leads to a rise curve resembling the rise curve c in FIG. 2. This practice minimizes the void size and the void frequency and the thickness of the overroll layer (i.e., the thickness of the panel edge layer in which the cells are chaotically arranged) is not very pronounced.
Such a compromise is technically still reasonably feasible with slowly reacting plastics, such as some polyurethane foams (PUR foams), in which the foam rises relatively slowly. However, the compressive strengths and insulating effects in the PUR foams produced in this way are still not always completely satisfactory.
With highly reactive plastics, such as polyisocyanurate foams (PIR foams), this compromise is however no longer technically reasonably feasible. Due to the extremely rapid rise of the PIR foam, a rise curve resembling the rise curve b in FIG. 2 occurs quasi-automatically. A process window for a production similar to the rise curve c in FIG. 2 is not possible in practice and a procedure corresponding to the rise curve a in FIG. 2 has to be avoided in principle on account of the formation of voids.
With PIR foams, until now, one has had to be satisfied with compressive strengths and insulating effects that do not correspond to the optimum values of this material.
EP-A-689 920 describes a process and an apparatus with which rectangular slabstock foams or foam sheetings can be produced from a liquid foam or from a foamable reaction mixture. This disclosure discusses the problem of on the one hand distributing the foamable mixture uniformly over the width of the transporting belt, and on the other hand of preventing air inclusions when the upper laminating film is applied.
EP-A-689 920 teaches the use of a deflecting element (i.e., in principle a calibration plate) with which the mixture can be distributed, and positioning this deflecting element in the transporting direction so that the rise of the foam takes place only after the exit of the foamable mixture from the channel between the deflecting element and lower belt. This means that even if a liquid foam (froth) is used as the reactive mixture, the actual rise process from the chemical reaction only starts behind the deflecting element.
The process and apparatus described in EP-A-689 920 are ideally suited for the production of slabstock foam. In this case, block heights of 1 to 2 m are in fact produced.
Serious defects are found, however, in the production of panels according to the process of EP-A-689 920. The panels have in fact only a thickness of 0.01 to at most 0.2 m, resulting in completely different production conditions.
When using unfrothed reactive mixture, the distance between the deflecting element (e.g. calibration plate) and lower belt must be of an order of magnitude of about 0.3 to 8 mm. This can be achieved only with an extremely high technical effort, particularly for thin panels, since the distance of, for example, 0.3 mm must be exactly maintained over the whole surface of the deflecting element. If this is not the case, large density differences occur in the panel, which in turn leads to distortion and thus to rejects.
When using frothed reactive mixture this problem is less serious, since in this case the distance between the deflecting element and the lower belt is larger, corresponding to the froth portion.
However, another serious problem arises in this case. When the pre-frothed reactive mixture flows through the calibration channel, cells in the froth are destroyed due to the friction, which then collapse and lead to the formation of gas pockets. Thus, although in this case air inclusions are indeed avoided, new gas pockets are produced.
Further, EP-A-689 920 does not disclose that in panels the cells have to be vertically arranged relative to the outer layers in order to obtain optimal compressive strength and optimal insulating effect or how this is to be achieved.